Liquids, such as petroleum or other fuels are often transported in metal tanks and/or through metal pipes. For example, petroleum fuels may be transported in metal tanker trucks and transferred to and/or from other tanks through a suitable passage or tube such as a pipe or hose. The use of tubes fabricated from opaque materials, such as metals or rubbers, often precludes direct observation of the flow and/or type of fluid through the tube. Inclusion of a transparent region in the tube may permit a direct observation of fluid flow through the tube. Further, in the case of fuels, the type of which may sometimes be distinguished by color, such a transparent region in the tube may permit a direct observation of the type and/or color of the fuel present in and/or flowing through the tube.
Examples of sight glasses and pipes incorporating the same are disclosed in U.S. Pat. Nos. 5,125,269; 5,628,231; 4,888,990; 1,886,444; and 1,042,730. The disclosures of these and all other publications referenced herein are incorporated by reference in their entirety for all purposes.